1. Field of the Invention
The invention relates to a clock generator for a pulse repeater of a receiver in a pulse transmission system.
2. Prior Art
A clock generator of this type is known from the U.S. Pat. No. 4,251,886. The clock generator of this publication is included in a pulse repeater of a receiver in a pulse transmission system more specifically having transmission channels with a very strong bandwidth limitation, i.e. a bandwidth smaller than the Nyquist frequency of the pulse signals, which type of transmission system the invention is aimed at.
The problem of pulse synchronization in these pulse transmission systems is solved by co-transmitting in the pulse signal band a pilot signal which is derived via a fixed relationship from the pulse repetition frequency and which can be reconverted thereto. In the known clock regenerator, use is made of a pilot signal superposed on the transmitted pulse signal to generate clock pulses in the pulse repeater which are applied to the clock input of one or more pulse regenerators.
When employing the known techniques, the use of a pilot signal is compulsory as it is not possible with the known techniques to recover the clock pulses from the transmitted pulse signal itself, because with these techniques the clock information is recovered from the part of the signal spectrum lying around the Nyquist frequency and this part of the signal spectrum has already been suppressed in case of a transmission channel having a strong bandwidth limitation.
In synchronization techniques using pilot signals, strict requirements are made on filters which have to separate the pilot signal from the information signal, the transmitted pulse signal. In addition, this type of clock recovery is strongly dependent on the real random nature of the information signal as each spectral line in the neighbourhood of the frequency of the pilot signal may cause erroneous synchronization. Consequently, when tests and measurements are carried out, due care should be taken and pseudo-random sequences of very great word length should be used, which sequences have to be generated especially for this purpose.
In addition, in non-linear transmission channels it is not even possible to superpose a pilot signal on the information signal; for example this is the case with digital magnetic recording or modulation of a non-linear laser.